Recently, higher performance steel is demanded than ever with the enlargement of the structures or the weight reduction of the automotive parts. In addition to this, upon the production of the steel, resource saving and energy saving are also important issues. Then, upon the production of the steel, it is desirable to be capable of producing the steel by production processes less than the conventional production processes, without installing more or newly establishing facilities.
Conventionally, a number of high-strength steel sheets have been developed. For example, Patent Literature 1 discloses a technique related to an automotive cold-rolled steel sheet in which high strength and high ductility are achieved at the same time and press formability and impact energy absorbing capability are excellent. The cold-rolled steel sheet is a thin steel sheet in which strength is increased with refinement of ferrite crystal grains by suppression of the amount of expensive alloy elements to be added and the balance with ductility, which is important for press formability, is excellent. In a production process of the cold-rolled steel sheet, after hot rolling, cold rolling is performed and appropriate annealing is performed. However, according to this technique, expensive alloy elements such as Mo and Ni are added as an essential additive element, although being a small amount; and it is necessary to perform the annealing process on the thin steel sheet subjected to the rolling.
Further, Non Patent Literature 1 discloses a steel sheet (referred to as NewTRIP steel) in which steel similar to low carbon steel having a chemical composition of 0.1% C-5% Mn-2% Si with a high content of Mn and Si without the addition of expensive alloy elements is used and a work hardening exponent is increased during a low-temperature reheating treatment after annealing by the high content of Mn which increases the fraction of residual austenite, by the high content of Si which suppresses the generation of cementite, and by C which is discharged from ferrite to austenite and stabilizes the residual austenite. In this process, however, the thin steel sheet is required to be subjected to complex processes such as the annealing treatment and the low-temperature reheating treatment after being subjected to rolling, and a problem of process efficiency is not solved in terms of energy saving. Since the thin steel sheet is referred to as production target steel, a cold rolling process is also essential in addition to a hot rolling process.
Meanwhile, several types of high toughness steel to be used in structures or the like except for the thin steel sheet have been also developed as production target steel. For example, Patent Literature 2 discloses a technique related to high-strength steel in which delayed fracture resistance is excellent with high strength and high ductility and toughness is dramatically improved. According to this technique, steel is exemplified in which tensile strength is 1660 to 1800 MPa, elongation (total elongation) is 18.5 to 19.2%, and impact absorption energy of a V-notch Charpy test at a room temperature is 305 to 382 J/cm2 (see Examples 1 and 17 indicated in Table 6 in Patent literature 2). However, even in this technique, expensive Mo of about 1.0% is contained as a chemical composition, and after any one of annealing, tempering, and aging is performed as a production process under predetermined conditions of temperature and time, working (warm working) is required at a temperature of 350° C. or higher but (AC1−20° C.) or lower.
Furthermore, as techniques proposed by the present inventors, there are Patent Literatures 3, 4, and 5. Here, the structure of the steel disclosed in Patent Literatures 4 and 5 differs from the intended structure (martensite) of the present invention in terms of being two-phase structure of α/γ. In addition, mechanical properties of the steel disclosed in Patent Literatures 4 and 5 also differ from those of the martensitic steel of the present invention in that the strength of the present invention is relatively high and the ductility is low compared to the α/γ structures.
In addition, since Patent Literature 3 discloses martensite structure steel, structures or mechanical properties of the steel are similar to those of the martensitic steel of the present invention. However, since the composition of the steel disclosed in Patent Literature 3 has the range of 0.05 to 0.2% of C in terms of the composition, there is a problem that tensile strength TS is only a level of 1400 MPa due to the very low content of C, and thus it is not possible to obtain tensile strength of a level of 2000 MPa which is an index. In addition, in view of characteristics of the steel disclosed in Patent Literature 3, there is a general nature of carbon steel that the ductility deteriorates as the high strength increases.
As described above, according to the techniques disclosed so far, the problems of resource saving and energy saving are not solved, and a large load is applied to a working apparatus in a normal production line for carrying out warm working at a relatively low-temperature range, so that there is a problem in industrially wide use.
In addition, when there is a demand to change the level of high strength, the level of high strength can be easily changed by increasing or lowering of the content of C. However, there is a contradictory problem that the ductility is lowered when the strength is increased by the increasing of the content of C, whereas the strength is lowered when the ductility is increased by the lowering of the content of C.